In recent years, various attempts have been made to solve problems associated with fuel consumption and environmental pollution in the transportation industry, including automobiles, aircraft and high-speed electric trains. Under these circumstances, weight reduction has increasingly gained importance. Aluminum and magnesium are widely used at present as materials for weight reduction of transportation vehicles.
Hot extrusion is typically used to produce aluminum- and magnesium-based components and parts. Hot extrusion is very advantageous in terms of production cost because highly accurate and complex molded articles can be produced through only one thermal deformation step.
Although high-strength, difficult-to-process aluminum alloys, typified by A2XXX and A7XXX, Mg—Al—Zn-based alloys, high-strength Zr-based ZK60 alloys and copper-containing ZC63 alloys have higher specific strength and higher specific stiffness than other aluminum and magnesium alloys, their productivity is one fifth or one sixth of that of the other aluminum and magnesium alloys. Greatly reduced extrudability is responsible for the low productivity of the high-strength alloys. Further, the high-strength alloys drastically shorten the life of extrusion molding apparatuses due to high pressure at the initial stage of extrusion. Furthermore, the structure of molded metals produced by conventional hot extrusion processes is elongated in a predetermined direction and becomes anisotropic, resulting in low strength of the molded metals.
On the other hand, die casting and thixoextrusion molding are known to be useful for producing aluminum and magnesium alloys. According to a thixoextrusion molding process, a metal material is extrusion-molded in a temperature region where solid and liquid phases coexist. Thus, thixoextrusion molding is a new phase-change molding process that combines the inherent advantages of casting and forging processes.
However, thixoextrusion molding processes have the problem that semi-solid metals may be ignited during extrusion molding. Although protective gases are useful in inhibiting the ignition of semi-solid metals, they are harmful to humans, cause metallic equipment to corrode and have secondary problems, including global warming.
Metal bars and tubes can be produced using extrusion molding apparatuses. For example, a metal tube is produced by extruding metal in a solid state under a high pressure or using an extrusion molding apparatus having a porthole die. However, the high-pressure extrusion process involves a high loss of the raw material, and the porthole die extrusion process has a problem in that welding seams are formed.